This invention relates to a photometric amplifier circuit for amplifying an electric signal responsive to the quantity of light and, more particularly, to a photometric amplifier circuit adapted for exposure measurement in cameras.
In a photometric amplifier circuit of a high input impedance having a photo-current converting element connected between the inverting and noninverting input terminals of a differential or operational amplifier for amplifying an electric signal responsive to the quantity of light, a PN junction diode to be used as the photo-current converting element, using a semiconductor such as silicon, gallium arsenide, gallium arsenide phosphide, etc. exhibits a high internal resistance at times of low illumination or at times of little light quantity and, accordingly, forms a large time constant circuit with the high internal resistance and junction capacity of the diode itself. Generally, a negative feedback circuit is connected between the output terminal and the inverting input terminal of the differential amplifier. Accordingly, the potential of the noninverting input terminal of the differential amplifier generally tends to be increased in absolute value with respect to that of the inverting input terminal thereof in the transient state when power is supplied from a power supply due to the effects of the aforesaid time constant and negative feedback circuits. This potential applies an undesired accumulated charge to the photo-current converting element and latches up the output of the differential amplifier to a certain undesired value for some time after the power is supplied from the power supply. As a result, the photometric amplifier circuit does not amplify the detected current indicative of a low quantity of light and does not operate normally until this accumulated charge is removed from the converting element. That is, it lengthens the stand-by time from the time the power is supplied by the power supply to the converting element to the time the photometric amplifies amplifier normally. Such a latchup phenomenon occurs when the potential of the noninverting input terminal of the differential amplifier becomes higher than that of the inverting input terminal thereof when the power is supplied from the power supply, because of certain causes in addition to the aforementioned effects of the negative feedback circuit.
This latch-up phenomenon lengthens the stand-by time of the differential amplifier. When the photometric amplifier circuit amplifies a particularly low quantity of photo-current to control an optical instrument such as a camera by the output signal therefrom, the phenomenon lowers the performance of the optical instrument or makes precise control of the optical instrument impossible. In particular, in the automatic electronic exposure control of a camera, it becomes impossible to accomplish photometric control immediately after the power is supplied from the power supply to the photometric circuit by pressing a release button of the camera. This detracts from camera performance.
In order to better understand this problem, it will be described regarding an example of a photometric amplifier circuit according to the prior art shown in FIG. 1.
In FIG. 1, reference numeral 1 indicates a differential amplifier which has inverting and noninverting input terminals (-) and (+) and an output terminal (OUT). Reference characters Rf and Rs are resistors for forming the negative feedback circuit of the differential amplifier 1. PC indicates a PN junction photo-current converting semiconductor (photo-diode) inserted between a pair of input terminals of the differential amplifier 1. LD is a PN junction semiconductor diode for logarithmically compressing the current of the photo-current converting element PC. Reference numeral 2 is a standard voltage generating circuit for applying a standard voltage to the semiconductor diode LD. 3 is a DC power supply. S is a power switch for supplying the power from the power supply 3. The photodiode PC exhibits a high resistance in this circuit when illuminated by a low quantity of light. The equivalent circuit of this circuit has, as shown in FIG. 3, a photocurrent source Id, a PN junction capacity Cd, and a high resistance Rd.
If the potential of the noninverting input terminal (+) of the differential amplifier 1 becomes higher in absolute value then that of the inverting terminal (-) thereof when the power switch S is closed, the output voltage OUTPUT of the output terminal OUT of the differential amplifier 1 is latched up to an undesired output voltage exceeding a normal output voltage Vs corresponding to a low quantity of light for the time t.sub.1 when it becomes the normal output voltage Vs until it decreases from a high output voltage Vh along a discharging time constant circuit and stabilizes to the voltage Vs, as shown by a curve l.sub.1 in FIG. 2. The standby time is lengthened to t.sub.1 in such a photometric amplifier circuit. The differential amplifier 1 amplifies the voltage difference applied between both the input terminals thereof by the gain thereof as it is, as indicated by the broken lines. Since the output voltage cannot, however, become higher than the power voltage Vcc, it is latched up to the upper limit value Vh of the output voltage of the differential amplifier 1.
To shorten the long stand-by time, one approach that has been considered is to accelerate the discharging of the accumulated charge of the photodiode PC by inserting the PN junction semiconductor diode BD so as to provide a bias opposite to the generated standard voltage of the standard voltage generating circuit 2 and by clamping the voltage at the noninverting input terminal of the differential amplifier 1 to the standard voltage when the power is supplied from the power supply. However, the stand-by time becomes as designated at t.sub.2 even by this method, as indicated by a curve l.sub.2 in FIG. 2. Thus, the latch-up phenomenon of the amplifier circuit cannot be completely eliminated.